Rainbow Trout (Oncorhynchus mykiss)
Region has an influence on many aspects of this fish, which is found both in the ocean and in fresh water. Its colour, size and spawning schedules are all determined by where it lives. This species is considered one of the top five sport fishes in North America—and the most important sport fish west of the Rocky Mountains.
Occurs in Canada from the Avalon Peninsula of Newfoundland across the southern parts of the provinces from Nova Scotia to Ontario; also occurs in the prairies and in interior BC as well as in the eastern Pacific ocean
About the rainbow trout
Elongate, fusiform body shape with 60-66 vertebrae, 3-4 dorsal spines, 10-12 dorsal soft rays, 3-4 anal spines, 8-12 anal soft rays, 19 caudal rays. Adipose fin present, usually with black edge. No nuptial tubercles but minor changes occur to the head, mouth and colour in spawning males. Coloration blue to olive green above a pink band along the lateral line and silver below. Back, sides, head and fins covered with small black spots.
Colouration varies with habitat, size, and sexual condition. Tendency for stream residents and spawners tend to be darker with more intense colour, whereas lake residents are brighter and more silvery. Absence of hyoid teeth is the most easily distinguishing characteristic from cutthroat trout.
Rainbow trout and steelhead are the same species of fish—but while steelhead are anadromous (they migrate up rivers from the sea to spawn), rainbow trout live in lakes and rivers. Anadromous forms of the trout can convert to resident populations when droughts occur or river dams block access to the ocean. And resident populations can become anadromous if ocean access becomes available. It is typical to have both life history patterns occurring in the same stream.
The ocean variety has a dark blue back and silver sides and belly. In non-migratory rainbow trout the back is bluish to olive green, silvery green on the sides and white over the belly. The stripe that runs along the side of the fish—from head to tail, ranges in colour from light pink to vivid red or a reddish purple. The stripe is more pronounced in mature fish and especially in breeding males. Small dark spots dot the sides, tail and dorsal fins.
Stream-caught rainbow trout usually weigh under 0.45 kg, while fish from the rivers and lakes weigh between 0.91 and 2.27 kg. Rainbows that have migrated to sea or large inland lakes such as steelhead of the Great Lakes can weigh 6.8 to 9.07 kilograms upon return, although most weigh between 3.6 and 4.0 kilograms.
Rainbow trout eat invertebrates (plankton, larger crustaceans, insects, snails and leeches). Depending on their size and habitat location, they also eat other fishes and fish eggs. Larger bottom crustaceans and organisms such as Gammarus can be part of their diet, as can larvae and virtually all aquatic insects that live in the fish’s habitat.
Habitat and Biology
The rainbow trout is a hardy fish that is easy to spawn, fast growing, tolerant to a wide range of environments and handling, and the large fry can be easily weaned on to an artificial diet (usually feeding on zooplankton). Capable of occupying many different habitats, ranging from an anadromous life history [strain known as steelhead] (living in the ocean but spawning in gravel-bottomed, fast-flowing, well-oxygenated rivers and streams) to permanently inhabiting lakes.
The anadromous strain is known for its rapid growth, achieving 7-10 kg within 3 years, whereas the freshwater strain can only attain 4.5 kg in the same time span. The species can withstand vast ranges of temperature variation (0-27 °C), but spawning and growth occurs in a narrower range (9-14 °C). The optimum water temperature for rainbow trout culture is below 21 °C. As a result, temperature and food availability influence growth and maturation, causing age at maturity to vary; though it is usually 3-4 years.
Females are able to produce up to 2 000 eggs/kg of body weight. Eggs are relatively large in diameter (3-7 mm). Most fish only spawn once, in spring (January-May), although selective breeding and photoperiod adjustment has developed hatchery strains that can mature earlier and spawn all year round. Superior characteristic selection is also achieved by cross breeding, increasing growth rates, resistance to disease, and prolificacy, and improving meat quality and taste. Genetic manipulation of the embryo sex chromosomes producing sterile, triploid females, hence avoiding the ‘hook-like’ jaw that does not appeal to the customer, and ensuring that introduced/escaped individuals cannot breed.
Trout will not spawn naturally in culture systems; thus juveniles must be obtained either by artificial spawning in a hatchery or by collecting eggs from wild stocks. Larvae are well developed at hatching. In the wild, adult trout feed on aquatic and terrestrial insects, molluscs, crustaceans, fish eggs, minnows, and other small fishes, but the most important food is freshwater shrimp, containing the carotenoid pigments responsible for the orange-pink colour in the flesh. In aquaculture, the inclusion of the synthetic pigments astaxanthin and canthaxanthin in aquafeeds causes this pink colouration to be produced (where desired).